1. Field of the Invention
This invention pertains to an apparatus for treating a cylindrically-shaped element, and a process for positioning the cylindrically-shaped element in the apparatus. In particular, the apparatus includes a means for moving the element to a pre-defined position for treatment. The cylindrically-shaped element is moved and positioned for treatment in the apparatus, creating a printing form.
2. Description of Related Art
Flexographic printing plates are well known for relief printing on a variety of substrates which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminium foils, etc. Flexographic printing plates can be prepared from photosensitive printing elements containing a layer of a photosensitive composition such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive elements generally have the layer of the photopolymerizable composition interposed between a support and a cover sheet or multilayer cover element. Upon imagewise exposure of the photosensitive element to actinic radiation, photopolymerization of the photosensitive composition occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. Conventionally, the element is treated with a suitable solution to remove areas of the photopolymerizable layer that were not exposed and leaving a printing relief which can be used for flexographic printing.
As an alternative to solution development, a “dry” thermal development process may be used which removes the unexposed areas without the subsequent time-consuming drying step. In a thermal development process, the composition layer, which has been imagewise exposed to actinic radiation, is contacted with an absorbent material at a temperature sufficient to cause the composition in the unexposed portions of the photosensitive layer to soften or melt and flow into an absorbent material. See U.S. Pat. No. 3,060,023 (Burg et al.); U.S. Pat. No. 3,264,103 (Cohen et al.); U.S. Pat. No. 5,015,556 (Martens); U.S. Pat. No. 5,175,072 (Martens); U.S. Pat. No. 5,215,859 (Martens) and U.S. Pat. No. 5,279,697 (Peterson et al.). The exposed portions of the composition layer remain hard, that is, do not soften or melt, at the softening temperature for the unexposed portions. The absorbent material collects the softened un-irradiated material and then is separated and/or removed from the composition layer.
Processors for thermal development of flexographic printing elements are known, for example, from U.S. Pat. No. 5,279,697 and U.S. Pat. No. 6,797,454. In both thermal processing apparatuses an irradiated photosensitive printing element comprising the support and the composition layer is mounted on a drum and a continuous web of absorbent material is passed over a hot roll. The hot roll is urged towards the drum pressing the web against the photosensitive element and forming a nip. Heat is transferred by conduction from the hot roll, through the absorbent web, to the photosensitive element upon contact so the temperature of the composition layer is raised sufficiently to enable the unirradiated portions of the composition layer to liquefy and be absorbed into the absorbent material. The heat transfer might be assisted by IR radiation using an additional IR heater directed to the surface of the composition layer. As the drum and hot roll rotate in contact together, the web is pressed against the photosensitive element to absorb the liquefied unirradiated composition and is then separated from the element. After the unirradiated composition is removed the resulting element has a raised relief surface of hardened areas that is suitable use as a printing plate.
Besides flat or planar photosensitive printing elements, it is also possible to thermally process cylindrically-shaped photosensitive printing elements, such as seamless photopolymer sleeves, or so called plate-on-sleeves. Seamless photopolymer sleeves include at least a continuous or substantially continuous layer of the photopolymerizable composition on a cylindrical support. Plate-on-sleeves include a flat photosensitive printing element mounted onto a cylindrical support.
Development by commercial processors of cylindrical photosensitive printing elements having different diameters can be difficult, time consuming, and expensive. However, an apparatus and process for thermal development of cylindrical photosensitive printing elements is disclosed in the United States Patent Application Publication 2006/0104675 (EP1657593), which overcomes these problems. The process treats a photosensitive element having a cylindrical support and a composition layer adjacent the support opposite an interior surface of the support, the composition layer capable of being partially liquefied. The apparatus includes first means for supporting the photosensitive element by contacting a first part of the interior surface of the cylindrical support; second means for supporting the photosensitive element by contacting a second part of the interior surface of the cylindrical support different from the first part, wherein the cylindrical support has one or more unsupported parts between the first part and the second part; and means for treating an exterior surface of the photosensitive element opposite the support to form a relief surface in the element. The cylindrical photosensitive element is not fully supported during thermal development. It is only partially supported by the first support means and the second support means of the apparatus. The cylindrical photosensitive element is not firmly fixed on a supporting cylinder, such as a drum, during thermal processing, but it is processed in a so-called “loose-fit” mode. In loose-fit mode, the interior surface of the cylindrical support of the photosensitive element is not fully contacted or supported by a supporting cylinder; and only a part or parts of the interior surface are in contact with or supported by a supporting means. In some cases, the hot roll is urged towards the support means pressing the absorbent material against the cylindrically-shaped printing element and forming a nip. Alternatively, the supported photosensitive element is brought into contact to a fixed heated roll by moving the loose-fit structure into a controlled impression.
A problem sometimes arises when the cylindrical photosensitive element has an axial length less than an axial length of the means for supporting the element in the apparatus, in that the position of the photosensitive element on the support means can influence the uniformity of pressure applied to the along the axial length of the photosensitive element at the contact zone of the nip. It is desirable to assure pressure uniformity on the photosensitive element across the nip since non-uniform application of pressure can contribute to the resulting relief structure of the printing form having non-uniform relief depth. It is desirable to avoid differences in pressure at the nip across the axial length of the processed cylindrically-shaped printing element and to create or maintain a symmetrical pressure profile. Also, the mounting cylindrical printing elements having the axial length less than the axial length of the means for supporting oftentimes causes an operator to insert his arm well into an opening of the apparatus. In some cases, the operator may even need to have some scale or ruler, and most probably a special jig, to move the cylindrical printing element sufficiently into the apparatus. There are some issues associated with the mounting of cylindrical printing elements, particularly those printing elements having its axial length less than the axial length of the support means, into the apparatus including: operators might not have long enough arms to move the printing element into the desired position; operators may touch hot surfaces, such as the heated roll or IR heater compartment, within the apparatus; operators may touch sharp edges and corners of the apparatus; and, operators may experience smells or chemical residues that may emit from inside the apparatus while the operator is moving the element. The removal of the cylindrical printing element after processing has the same associated set of problems.
Therefore, there is a need to overcome the problems of related art and to improve existing processors for thermal development of cylindrically-shaped printing elements. In particular there is a need for a solution that ensures a safe and quick positioning of cylindrically-shaped printing elements of different length.